The present invention relates to a paper for receiving toner images in electrophotography. More particularly, the present invention relates to a paper that is capable of producing copies with high image quality in monochromatic or color electrophotographic copiers that are adapted for reproduction of images comparable to those attained by printing process.
Coated papers such as art paper are currently used in multicolor printing and other sophisticated areas of printing which chiefly depend on offset lithography. This is primarily due to the high level of surface smoothness of coated papers. This affords such advantages as more accurate and complete image reproduction because of the intimate contact between the paper and the ink film formed during printing, and good quality color reproduction because of high gloss of the image.
The adaptability of coated papers for electrophotography has been discussed by Tsukatani et al., who stated as follows: in physical terms, coated papers have smooth surfaces and will contact uniformly to an electrophotographic photoreceptor, so they would theoretically yield toner images having good aesthetic appeal after the transfer step. In fact, however, they have produced undesirable grainy images when used in existing copiers. Tsukatani and Ohta; "Paper for Non-impact Printers", Kami Pulp Gijutsu Times, 27, No. 4, 31-36, 1984. As a matter of fact, coated papers developed for printing are seldom used in electrophotography because they have none of the advantages normally associated with good image quality.
Independently of the review by Tsukatani et al., the present inventors have found that smooth-surfaced coated papers for printing will blister when toner images are fixed thereon during an electrophotographic process. This problem was found to occur irrespective of the fixing method employed, such as heated rolls or an oven. This is because the air permeability of the coated papers is too small to prevent the paper layer from peeling off the base sheet as a result of the thermal expansion of the water vapor in the paper.
Tsukatani et al. also reviewed a method for producing high-quality images in electrophotography and suggested, on the basis of comparison with conventional priting processes, certain approaches characterized by the use of finer toners or the formation of a thinner layer of transferred toner particles. Tsukatani and Ohta; "Paper for Non-impact Printers", Kami Pulp Gijutsu Times, 27, No. 3, 45-49, 1984. In this respect, the use of smooth-surfaced coated papers would afford certain advantages associated with image quality, if the problems encountered with coated printing papers in the electrophotographic process could be solved. It has also been found that smooth-surfaced coated papers used in electrophotography produce better results in the fixing step specifically intended for providing high image gloss and good color reproduction.
Paper for receiving a single-component magnetic toner images has been proposed for use in electrophotography. This paper has properties which are extremely close to the properties of coated papers (see Japanese Patent Publication Nos. 26026/1983, 24916/1982, 53592/1982 and 55139/1982).
To eliminate the formation of defective images during the transfer step resulting from generally low electrical resistivity, this paper is prepared by coating raw paper with coating color containing high-resistivity resin as a main component. However, in order to attain an appearance resembling that of plain paper, the coating weight has to be relatively small and the surface of the resulting paper is therefore insufficiently smooth to be usable in electrophotography for producing high-quality images. Although the coating applied to form this paper contains the same pigments as those employed in coatings used in the manufacture of coated printing paper, the pigment loading in the former case is much smaller than in the latter case, where pigments are used in large amounts chiefly for the purpose of providing smooth surfaces. There are several principal reasons for this small pigment loading in this paper. First, in the manufacture of electrophotographic image transfer paper, pigments are used in such amounts as to provide an appearance resembling that of plain paper. If the pigments are used in large amounts, the effect of the high-resistivity resins used may be impaired. Therefore, a level of surface smoothness that is comarable to that of coated printing paper is not attainable solely by increasing the coating weight. Furthermore, increased amount of coating weight can lead to "blocking" problems, which must be eliminated in order to produce paper suitable for use in an electrophotographic process as image receiving sheets.
An object, therefore, of the present invention is to solve the aforementioned problems of the conventional art and to provide an electrophotographic image transfer paper that will not blister during the fixing step and is capable of consistently producing copies with high image quality.
Another object of the invention is to minimize problems during feeding of paper to a copier caused by static friction between adjacent sheets.
Additional objects and advantages will be obvious from the description which follows or may be learned by practice of the invention.